The present invention relates to a flexible floating boom comprising transverse stiffeners of variable stiffness.
This boom is in particular suitable for defining shipping zones, or water areas which may be used for aquatic activities such as, for example swimming, or for aquaculture.
A very important application of the invention is for fighting against water pollution caused by floating solid wastes or by chemical products, more particularly hydrocarbons.
The boom according to the invention may be used in a stationary position, stretched between two mooring points, or as a net or trawl secured at its ends to one or several tug boats in order to clean a polluted water area.
A first type of already known floating boom comprises a cylindrical buoyant member to which is secured a skirt weighted by a chain at its lower end. Traction loads exerted on such a boom are applied to the weighting chain or to a cable secured to the boom. A drawback of this type of booms is that in a strong current the skirt is hollowed and water rises on the upstream side of the boom, thereby reducing the boom height above the water level. The polluting products then may flow over the cylindrical buoyant member. When the water current is very strong the boom may become entirely flooded, with water passing over the boom.
A second boom type comprises a flexible structure forming a barrier provided with transverse stiffeners to which are secured floats adapted to support this flexible structure in a substantially vertical position in water so as to form a screen. Such booms have the advantage of keeping a substantially constant height above the water level, but as the strength of water current increases they are tilted and may even turn upside down.
Steadiness of such a boom can be improved either by providing it with weighting means, but this has the drawback of increasing its inertia and consequently impairs its response to wave action. Alternatively, the steadiness can be improved by increasing the distance between the floats and the barrier plane, so as to increase the uprighting torque, with these floats being placed on a single side or on both sides of the barrier.
It is also known that above a certain value of the current speed (about 1 knot), the booms can no longer contain the polluting products which then flow under the boom as a result of a vortex effect (driving of the polluting layer under the boom in the form of small droplets which rise to the water surface downstream of the boom).